1. Introduction
Fused Deposition Modeling (FDM) technology is just one from many technologies which are covered with Additive Manufacturing (AM) process. It is possible to produce components from metallic materials, from resins and also from polymers [
1,
2,
3]. Fused Deposition Modeling is the most wide-spread technology. Is very simple and also not so expensive as others. Using plastic wire as input material which is semi-melted and then by nozzle is deposited layer by layer to the required shape, defined by 3D digital mode [
4,
5,
6]. Very often are used materials as PLA (polylactic acid), ABS (Acrylonitrile butadiene styrene), PETG (Polyethylene terephthalate glycol), Nylon, PC (Polycarbonate) or many others [
7,
8,
9].
As composite materials we can see for example wood composites, where in basic polymer (mentioned above) are mixed wood particles. There is mostly up to 40% of wood particles [
10].
The same way are produced also the composites with brass particles, copper particles, bronze particles and others. Modern and innovative filaments with progressive properties are created with nanoparticles of different composition [
11,
12,
13,
14,
15].
The following article is focused on experimental investigation of the properties of the material Conductive PLA, which is designed for Additive Production using FDM (Fused Deposition Modeling) technology. This is material from Protopasta company (Vancouver, WA, USA). It consists of a basic matrix of thermoplastic Naturework 4043D PLA (Polylactic acid). The base matrix contains particles of Carbon Black material, which provides a specific property, electro-conductivity. Electro—conductive composite materials attract considerable industry attention, especially for their wide range of applications. By adding conductive fillers to the base matrix, it is an effective way to achieve such exceptional polymer properties. Carbon Black is now widely used in industry due to its low price, low weight and wide and easy availability [
16,
17,
18]. To achieve conductivity, it is necessary to use a relatively high content of carbon black in the base material, which can have a significant effect on strength, flexibility, but also other material properties [
19,
20,
21].
Carbon partcles could be used in such applications basically in three types of carbon fillers. Carbon Black (CB), Carbon Fiber (CF) and Carbon Nano-Tubes (CNT) [
22]. They are used as fillers in basic polymers. Can be used alone or in combinations. Carbon fibers have been widely used recently, mainly because of their greater availability, unlike carbon nanotubes, which are also more expensive and less affordable [
23,
24,
25]. Different structures, morphology and shapes of these fillers, their dispersion and other properties affect the conductivity of the prepared materials [
26,
27,
28,
29,
30,
31]. Carbon Black is the most commonly used filler due to its low cost, low density and good internal conductivity. Some studies suggest that the morphology of CB aggregates in a polymer matrix is grape shape like (
Figure 1), consisting of many individual CB particles with an average diameter of tens of nanometers [
32,
33,
34,
35].
The size of the aggregates and the distance between them are crucial for creating electrical conductivity [
36,
37,
38,
39]. The distances between the nearest adjacent multiparticulate surfaces must be narrow enough to create suitable conditions for the passage of electric current. The average inter-particle distance should be from 10 to 28 nm [
15].
Figure 2 shows 3 types of Carbon Black, Carbon fibers and Carbon nanotubes.
Such a material as filler could be used within basic thermoplastic mentioned above. When using the fillers, material properties are changed. Increasing the Carbon Black content within the matrix increasing the conductivity but on the other hand decreasing the strength of final product. Following part of this paper will deal with experimental determination of material properties of Conductive PLA material.
2. Materials and Methods
As mentioned above, the experimentally tested material is Conductive PLA. This material consist of Naturework 4043D PLA (Vancouver, WA, USA), thermoplastic and Carbon Black particles. The weight ratio of carbon black is about 25%. Material is in filament form, and is quite flexible, and is compatable with any PLA printing printer. Based on information from producer, Protopasta Conductive PLA is a good choice for low-voltage circuitry applications, touch sensor projects, and using prints to interact with touch screens, which require low conductivity.
The other resistance properties stated by producer are:
Volume resistivity of molded resin (not 3D Printed): 15 ohm-cm
Volume resistivity of 3D printed parts perpendicular to layers: 30 ohm-cm
Volume resistivity of 3D printed parts through layers (along Z axis): 115 ohm-cm
Resistance of a 10 cm length of 1.75 mm filament: 2–3 kohm
Resistance of a 10 cm length of 2.85 mm filament: 800–1200 ohm
Filament is in 1.75 mm diameter, supplied on spool. Other setting of 3D printer is very similar to conventional PLA filament.
As mentioned above, there will be two types of experiment. One is for tensile strength testing. Second for testing of resistance with different influencing factors change.
2.1. Tensile Strength Measurement
For tensile strength measurement, there were produced testing specimens (
Figure 3), designed with following of ISO standards ISO 527-1.
Before the specimens were produced on FDM 3D printer, the design of experiment is prepared. First of all factors had to be stated. We selected the type of infill, to figure out which is better for this kind of material. Selected are Rectilinear and Honeycomb as the most used in the practice. Then we specified two layer thickness for producing od specimens. Selected are 0.125 mm and 0.25 mm. The last specified factor is infill volume. We selected 50% to figure out, how the decreasing of volume change the measured tensile strength. The second level of infill volume is 90%, because 100% is not possible with honeycomb infill. But for comparison we produced also the 100% infill, but not with all of others combinations. Selected factors and their levels are specified in (
Table 1). Full factor experiment have been prepared to take into the consideration all the combinations of factors and levels (
Table 2).
Based on described information, the specimens are produced. There have been produced 4 specimens with the same settings based on defined design of experiment. After that, all produced specimens were tested on universal testing device Inspekt Desk 5 kN (Hegewald & Peschke, Nossen, Germany) (
Figure 4). The maximum testing force for this device is 5 thousand newton.
2.2. Resistance Measurement
The primary goal conductive material experiment is to determine the resistance of this material. In this case we want to first of all figure out how is the resistance changing with length of produced samples. So length is the first factor for this experiment. We decided to produce specimens from the minimum length 10 mm, then 20 mm and with this 10 mm differentiation up to length 100 mm.
The second factor is printing nozzle temperature. The purpose if to figure out how the printing temperature affecting final resistance. If the more molted and deposited material have better or worst resistance. Depending on minimum and maximum printing temperature advised by producer, we selected 190 °C and 220 °C.
The last factor is temperature during the resistance measurement process. One measurement were made within normal room temperature 25 °C, and the second with much higher temperature 80 °C.
Selected factors and their levels are presented in
Table 3.
Based on this selected factors and levels, the full factor is prepared, with all of the combinations as is shown on
Table 4. For each combination 5 samples were produced. So each experiment is repeated 5 times.
For this purpose are produced very simple specimens (
Figure 5) with square cross-section and with specified length as defined in design of experiment. For measurement is used conventional digital multimeter PU510 with accuracy ±0.5%. Each measurement were repeated five time, to prevent some random errors.
4. Conclusions
From the information and results presented above, the following conclusions can be stated. From the previous experiments is known the strength of the classical PLA material at 90% filling of the internal volume of the sample is 48.63 MPa [
40]. Similar results are available also from similar research in the range 47 MPa to 53 MPa [
41]. The experiment carried out in this paper with Conductive PLA materials shows the highest achieved tensile strength value of 32.1 MPa. This means that Conductive PLA material achieves 66% of the strength of conventional PLA material without additives. The assumption that adding Carbon Black filler to the PLA matrix reduces its strength is confirmed. The results also show how changing the production settings of the parts affects their final strength.
From experiments concerning the measurement of the conductivity resistance of the Conductive PLA material, the following can be determined. The known linear dependence of the resistance on the length of the printed samples was confirmed. It can be seen from the graphs (
Figure 9) that if we produce samples with a higher melting temperature of the material during its production, we achieve a lower resistance. On the other hand, if we measure at a higher temperature, the resistance is higher. From the data obtained from an extensive experiment, it was possible to prepare regression calculation models for the calculation of conductivity resistance, without the need for its experimental determination. When comparing our outputs with other research related to electrical resistance of polymers, we can state that the values from other works that are focused on the electrical resistance of polymers are in the range of measured values from our research [
42,
43,
44,
45].